This invention concerns a method and an apparatus for effecting relative separation between desired and undesired agricultural products, e.g. between mould-carrying agricultural products and agricultural products free of such mould.
Although the invention is applicable to the sorting of a number of different agricultural products, e.g. maize, it is more particularly applicable to the sorting of oil stock ground nuts which are liable to be contaminated by a toxic mould which produces aflatoxin.
After the oil has been extracted from ground nuts, the material remaining may be formed into a cattle cake. However, if the said ground nuts have been contaminated by aflatoxin, the cattle cake made therefrom is similarly contaminated, and since aflatoxin is highly toxic, many countries have legislation which limits the permissible amount of aflatoxin in cattle cake. Attempts have been made to detoxify the cattle cake, e.g. by the use of ammonia, but the cost and the undesirable side effects of such detoxification have so far prevented its use.
It is therefore desirable to sort the ground nuts so as to remove those affected by aflatoxin. There is, however, a major problem in connection with the sorting of such ground nuts by the use of light-sensitive detectors which are responsive to reflectance from the ground nuts. This problem is that ground nuts used as oil stocks are generally not treated with the care given to ground nuts used in confectionery, and the shelling of such oil stock ground nuts has normally resulted in a high percentage of skinned, partially skinned and split nuts whose kernel is visible to the light-sensitive detectors. It is therefore normally impossible for the detectors to determine whether an high level of reflectance from a ground nut in visible light arises merely from the presence of the said mould or from the visibility of the kernel due to the mechanical damage which has been done to the nut during the shelling. It is extremely important, however, that the sorting process should not involve the rejection of nuts merely because they have been mechanically damaged, since this might well involve the rejection of 60% of the nuts sorted, which would not be economical.
In our British Patent Specification No. 993,063 there is disclosed a sorting machine for effecting relative separation between ground nuts affected by a toxic mould and those not so affected. The sorting machine is arranged to direct ultra-violet and infra-red light onto ground nuts being viewed by two detectors which are respectively responsive to light of different wavelengths. The outputs of the two detectors are compared so as to produce a comparison signal which controls an ejector device for ejecting ground nuts affected by said toxic mould. One of the detectors is responsive to infra-red light and is provided to determine the extent to which the skin of the ground nut has been damaged or removed. The other detector is responsive to the colour in which the said toxic mould fluoresces due to said ultra-violet light and is thus provided to determine the extent to which the ground nut is affected by the said toxic mould.
We have now found, however, that the determination of the extent to which the ground nut is affected by the said toxic mould is best achieved by the use of a detector which is responsive to light whose wavelength is in the range 6,000 to 15,000 A, and thus is responsive to red or infra-red light rather than to light of the colour in which the toxic mould fluoresces.
Moreover, in British Patent Specification, No. 993,063 the infra-red detector is used to determine the extent to which the ground nut is provided with a skin. We have now found, however, that all ground nuts which are not affected by a toxic mould are highly reflective in the red or infra-red irrespective of whether they have a skin, and consequently that it is most important that the detector which is provided to determine the extent to which the ground nut is provided with a skin is responsive to light in a part of the spectrum outside the red or infra-red.